The present invention relates to temperature-activated memory elements made of a shape memory alloy. More particularly, the present invention relates to a bifurcated memory element having at least a lead-attachment portion and a substantially uncontaminated shape-memory portion and a method of making such a bifurcated memory element.
Alloys exhibiting a shape-memory effect are well known. For example, alloys of nickel-titanium, gold-cadmium, iron-platinum, indium-cadmium, iron-nickel, nickel-aluminum, and others have been observed to exhibit shape-memory characteristics. These alloys are known to exhibit a shape-memory effect upon martensitic transformation from a parent phase to a martensitic, or reversely, from a martensitic to a parent phase. Many properties of such alloys are discussed, for example, in Shape Memory Effects in Alloys, edited by Jeff Perkins, 583 pages, Plenum Press (1975).
During development of the present invention, temperature-activated memory elements made of shape-memory alloys were observed to experience varying degrees of dysfunction after several temperature-activation cycles. Such dysfunction is characterized, in part, by an inability of the memory element to move to assume its predetermined shape during reverse martensitic transformation when heated to a predetermined temperature. It was experimentally determined that such dysfunction results from introduction of contaminants into the memory element. These contaminanta may come from, for example, an electrically conductive lead or the like which is cohered to the memory element to permit an electric flow so as to heat the memory element to its predetermined temperature.
Contamination of a memory element is thought to result from introduction of certain ions into the crystal lattice of the shape-memory alloy comprising the memory element during martensitic transformation. An electrically conductive lead, solder, or the like cohered (i.e., soldered or welded) to the memory element provides a source of said certain foreign ions. For example, ions of silver, cadmium, lead, iron, or other ions are thought to enter and "poison" the crystalline structure of the shape-memory alloyed mechanism, thereby damaging or otherwise weakening the shape-memory effect function of the memory element during martensitic transformation.
During martensitic transformation, nickel-titanium shape-memory alloys (nitinol) undergo a "second order transformation" having an undefined intermediate phase between the parent phase and martensite. The crystal lattice of such alloys provides an internal structure which is very susceptible to migration and diffusion of foreign ions. Reference is hereby made to F. E. Wang, W. J. Buehler, and S. J. Pickart, "Crystal Structure and a Unique `Martensitic` Transition of TiNi," J.Ap.Phys., 36 (1965); and F. E. Wang, B. F. DeSavage, and W. J. Buehler, "The Irreversible Critical Range in the TiNi Transition," J.Ap.Phys., 39 (1968) for descriptions of transformation characteristics and properties of nitinol.
Ionic contamination of such shape-memory alloyed mechanisms is thought to result in part, from a complete or partial migration of contaminant ions through the mechanism during martensitic transformation. Essentially, the contaminant ions enter the mechanism at a lead-attachment site and then migrate individually or by means of a "domino-type" effect through the entire mechanism. It has been observed in the development of the present invention that relatively small concentrations of such ionic contaminants in a mechanism are sufficient to damage or weaken the shape-memory effect function of the mechanism.
One object of the present invention is to provide a memory element configured to move to assume its predetermined shape repeatedly when heated to its predetermined transition temperature without experiencing significant functional degradation due to contamination.
Another object of the present invention is to provide a memory element cohered (soldered or welded) to a lead wire or the like which can still move to assume its predetermined shape repeatedly without experiencing significant functional degradation due to contamination when subjected to thermal cycling through the transformation.
Yet another object of the present invention is to minimize dysfunction of a memory element by controlling the introduction of contaminants into the crystal lattice of a selected shape-memory portion of the memory element so that contaminant concentration levels in the selected shape-memory portion are regulated.
Still another object of the present invention is to provide a method of acting upon a memory element to disrupt the crystalline structure of a selected portion thereof or otherwise alter the selected portion to form barrier means in the memory element for limiting the migration of selected ionic materials or other contaminants across the memory element.
According to the present invention, a memory element made of a shape-memory alloy is provided. The memory element includes first and second portions, each portion having a characteristic internal structure, and partition means for interconnecting the first and second portions. The partition means has an internal structure dissimilar to the internal structures of at least one of the first and second portions.
In preferred embodiments, the memory element further includes an electrically conductive lead and connection means for coupling the electrically conductive lead to the first portion. Through the lead, electrical energy is communicated to the memory element. This energy acts to heat the memory element to a predetermined transformation temperature.
Preferably, the first portion functions as a lead-attachment portion and the second portion functions as a shape-memory portion. The dissimilar internal structure is configured to block transmigration between the first and second portions of selected ions originally communicated to the first portion. The dissimilar crystal structure provides a barrier that is in a state that does not undergo martensitic transformation and thus is not conducive to ion migration but serves to provide a block preventing ions from migrating into the second portion. Preservation of the shape-memory effect in the second portion is one advantageous result of such an ion migration-blocking configuration in the partition means.
At the same time, the dissimilar internal structure is configured to provide means for communicating electrically energy from the first "lead-attachment" portion to the second "shape-memory" portion. Such energy acts to heat the second "shape-memory" portion to a predetermined temperature so that at least the second portion moves to assume its predetermined shape.
In use, the dissimilar internal structure forming the partition means is thought to filter certain ions moving from the first portion toward the second portion so that such ions are substantially isolated or otherwise contained in the first portion. Advantageously, such containment effectively limits to the first portion any degradation of the shape-memory effect function of the memory element that might occur due to ionic contamination. Thus, the substantially uncontaminated second portion is free to assume its "memorized" shape when heated to its memory temperature, even though the first portion may not function in quite the same way.
Also in accordance with the present invention, a method is provided of making a temperature-activated memory element. The method includes the steps of providing a mechanism made of a shape-memory alloy having a crystalline structure and exposing a selected portion of the mechanism to an energy source to divide the mechanism into first and second portions interconnected by the selected portion. The energy exposing step is continued for at least a predetermined period of time to disrupt and alter the crystalline structure of the selected portion to provide a dissimilar structure configured to block transmigration of selected ions between the first and second portions.
In preferred embodiments, the energy source is a laser. The method can further include the step of connecting an electrically conductive lead only to the first portion after the exposing and continuing steps to provide means for applying an electric current to the mechanism. The selected portion advantageously provides a partition or thermally-stressed zone intermediate the first and second portions to isolate substantially in the first portion selected ions communicated from the electrically conductive lead to the first portion. Such isolation aids in minimizing ionic contamination of the second portion, thereby substantially preserving the shape-memory effect of the alloy mechanism comprising the second portion.
Additional objects, features, and advantages of the invention will become apparent to those skilled in the art upon consideration of the following detailed description of preferred embodiments exemplifying the best modes of carrying out the invention as presently perceived.